docs: def check_convention_globals(self, ds):
          '''
          Check the common global attributes are strings if they exist.
  
          CF §2.6.2 title/history global attributes, must be strings. Do not need
          to exist.
  
          :param netCDF4.Dataset ds: An open netCDF dataset
          :rtype: list
          :return: List of Results
          '''
          attrs = ['title', 'history']
  
          valid_globals = TestCtx(BaseCheck.MEDIUM, self.section_titles['2.6'])
  
          for attr in attrs:
              dataset_attr = getattr(ds, attr, None)
              is_string = isinstance(dataset_attr, basestring)
              valid_globals.assert_true(is_string and len(dataset_attr),
                                        "§2.6.2 global attribute {} should exist and be a non-empty string" # subsection message
                                        "".format(attr))
          return valid_globals.to_result()